This invention relates generally to lasers, and, more particularly, to a free expansion nozzle for use within a gas dynamic laser.
Since the development of the first working lasers, considerable time and effort has been expended in the search for higher output laser systems. The possible applications of high power lasers are unlimited in the fields of communication, manufacturing, construction, medicine, space exploration, and defense. Unfortunately many obstacles exist to the attainment of high power systems. Most lasers have a low efficiency and therefore to obtain high power or high energy outputs, considerably more energy must be furnished to the system than is extracted. If this energy furnished is electrical, then the system cannot have a large average power and still be portable, as is desired in some cases. The relative size and weight of laser systems, and the availability of materials, have also introduced obstacles to their development.
The gas dynamic laser has grown out of the initial laser effort and is representative of one of the more sophisticated laser techniques which has the capability of providing very high power radiation output, due primarily to the large gas handling capability characteristic of such a system and due to the large quantity of energy which can be added to the gases flowing in such systems.
Gas laser operation requires that a population inversion be established between upper energy levels and lower energy levels of the lasing medium. One example of such a laser would be the carbondioxide-nitrogen laser. Recent experimental investigations into gas lasers have shown that photon emission necessary for laser operation may be achieved by the resonant transfer of energy, through collisions, from a first gaseous substances, designated the "energizing substance" such as vibrationally excited molecular nitrogen (N.sub.2), to a second substance designated the "lasing substance" such as carbon dioxide (CO.sub.2). These experiments have shown that nitrogen and CO.sub.2 may be fully mixed together, such as in a fully mixed gaseous plasma, while the substances in this mixture are raised to respective specific energy levels, favorable to laser emission, as a result of the electron collisions in an electronic plasma. Also, it has been known to combust a complex substance such as cyanogen so as to generate carbon dioxide and molecular nitrogen with the molecular nitrogen in a highly energized state favorable to energizing the CO.sub.2 for laser emission. In either case, it is necessary that the nitrogen have sufficient energy in its vibrational mode so as to impart a substantial amount of energy to CO.sub.2 in the 001 state, which is commonly referred to as the upper laser level for CO.sub.2 molecules. The very efficient energy transfer between the nitrogen and the carbon dioxide results from a near identity of the energy spacing of certain of the vibrational states of these two substances.
Thus, in the present state of the high power gas laser art, lasing (which is the coherent stimulated emission of quanta of light energy) of one substance results from that substance being brought to a high, nonequilibrium energy state as a result of collisions with an energizing gas excited to a vibrational energy level which closely matches an energy level of the lasing substance (i.e., the upper lasing level in CO.sub.2 ). Simply stated, at least one CO.sub.2 molecule which is present in a region of population inversion will spontaneously emit a photon with an energy equal to the difference between the upper laser energy laser energy level and the lower laser energy level for a CO.sub.2 molecule. This is a quantum of light energy which is reflected back and forth in the optical cavity. The photon will impinge on another CO.sub.2 molecule and cause a rapid, stimulated emission of a second photon. This photon is also reflected back and forth in the optical cavity, and so forth which brings about a continuing avalanche of stimulated photon emission, at the lasing wavelength. This sequence will occur nearly instantaneously so that lasing is established in say, nanoseconds. The useful laser output is derived by coupling light energy out of the oscillating and/or amplifying optical cavity.
Within the gas dynamic laser an optically active medium is created by rapidly expanding vibrationally excited gas mixtures through an array of two-dimensional nozzles. Typically, the nozzles array consists of a large number of small nozzle blades characterized by extremely small throat heights and isentropic contours. Each nozzle being critically machined to have a throat area of, for example, 0.012 inches, a length of 3 to 4 inches and a flow contour of 1 inch. Precision machining of the nozzle blades is therefore extremely costly and maintaining these critical tolerances during gas dynamic laser operation is difficult.